A user keeps telling me that he gets the mentoined warning. I don't beleve that this comes from the Manager but I don't have any proof for that. Therefor I am here to ask the pros -> Is this Message from the Manager? -> If so, how can I disable it?

If the Message is not from the Manager, It would be nice to get some suggestions to solve this Problem.

It's supposed to only show if you are leaving a page where you've edited the forms without saving. It's just some Javascript, and some versions of MODx don't quite get it right. It doesn't hurt anything, it's just annoying.

Not sure if this helps, but I saw this problem for the first time today on 2.2.10-pl.
I was using Articles. I had an article container open, I had created an article, saved it. Everything was fine. The save button was greyed out. So, I went to close the article container, and it gave me this error. Again the saved button was greyed out.

I changed some text and got the save button to light up, pressed save. Thought this would fix it, not luck. pressing close still gave me the error.

I finally noticed that somehow the article template field had gotten reset. It did not have a valid template selected. I selected the template, pressed save, and "Ta Da" close worked.

You may not be working with Articles, but maybe what MODX is doing, is that for some reason it thinks a required field is not entered, but it is not displaying the red text or message? Maybe if they just put a value in some field it will stop the error message. It might me a temporary work around until someone provides you a way to disable this or fix it.